Proof of Concept of an Advanced Sun Photometer for Planetary Applications

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Abstract

A lightweight, compact, highly reliable instrument designed to measure gas phase and particulate concentrations has been developed that is suitable for use in planetary environments. Current efforts have focused on validating the direct aerosol optical depth measurements. Efforts have been primarily focused on characterizing the instrument angular response, as well as validating measurements of aerosol optical depth, and water-vapor columnar abundance. Recent work has been aimed at refining the analysis procedures. Comparisons of the Advanced Sun Photometer AOD measurements have been made with AOD readings taken with a commercial Microtops II handheld sun photometer.
Background: Dust is one of the primary drivers in the Martian atmosphere, second only to carbon dioxide. Understanding the characteristics and distribution of Martian dust is not only essential to understanding the climatology and weather of Mars, but could provide data essential for the development of future manned and unmanned missions to Mars.
Previous surface observations of Mars have been largely performed by instruments developed for geological, navigational, and engineering applications. A sun photometer instrument dedicated to the measurement of Martian dust would considerably increase our understanding of the atmospheric physics and chemistry of the Martian atmosphere, providing a detailed knowledge of the composition and phase of atmospheric gasses, the size and distribution aerosols, as well as the upwelling and down welling radiative flux.
Continuous measurements throughout the diurnal and seasonal planetary cycles are needed, since aerosols in the Martian atmosphere vary spatially and temporally. Although planetary orbiters can be used to obtain these measurements, the revisit times/rates of these orbiters are limited by their orbital geometry.
Concept: the CCD array records both the diffuse light entering the cone as well as the direct solar beam. Most sun photometers incorporate a rigid tracking mechanism to follow the sun. The tracking mechanism may provide a point of failure during the entry, descent, and landing phase or later during the operational phase due to the harsh entry/planetary environment. Making these systems more robust usually requires increasing the instruments size, weight, and/or power requirements. Eliminating the need for a tracking mechanism can significantly reduce instrument weight while increasing system robustness.
The Advanced Sun Photometer described here will use an optical system to provide a hemispherical field of view, removing the need for a sun tracking mechanism. A CCD array is placed at the base of the optical system captures and records the light. Measurements at specific wavelengths are achieved by interposing various interference filters into the light path by means of a filter wheel.
The Advanced Sun Photometer can be adapted to other planetary bodies with sensible atmospheres.